FIG. 1 schematically shows an example of an optical modulator of Mach-Zehnder interferometer type, often called an MZI modulator. An optical wave having a power P0 arrives through a waveguide to an optical separation unit S1, a Y-separator here. The initial wave is separated into two half-power waves, respectively guided in two parallel, upper and lower branches. Each branch comprises, in series, a fast optical phase modulation diode HSPM (“High Speed Phase Modulator”) and a slow optical phase adjusting diode PINPM.
Each of the diodes HSPM and PINPM introduces an adjustable phase delay of the optical wave crossing the diode by producing electric charge in the light path. The HSPM diode operates in reverse bias mode; electric charge is pulled from the junction into the optical path by a junction depletion phenomenon, the junction being offset relative to the optical path. The diode PINPM operates in a forward bias mode. It includes a P-I-N junction, the intrinsic region of which is in the light path and receives electrical charge by an injection phenomenon.
The diode PINPM reacts slowly to changes in its bias, but it has a wide range of phase delay adjustment—it is used to adjust an optimal quiescent phase delay in the branch. Thus, the diodes PINPM of the two branches (PINPM1 and PINPM2) receive respective constant bias currents depending on the quiescent phase delays to be introduced in the two branches.
The diode HSPM reacts quickly, but has a low phase modulation amplitude—it is used to modulate the phase delay with a digital signal to be transmitted around the quiescent phase delay established by the diode PINPM. Thus, the diodes HSPM of the two branches (HSPM1 and HSPM2) receive voltage signals that are modulated, based on the digital signal to be transmitted, between 0 and a positive value Vb. The voltage signals applied to the diodes HSPM1 and HSPM2 are complementary so as to produce a differential effect in the two branches.
The two branches reach an optical junction unit J1, here a directional coupler. The optical waves incident on the two channels of coupler J1 are shifted by 180° at rest, whereby, in the case of a symmetric coupler, the optical power P1, P2 delivered by each channel of the coupler J1 is 50% of the input power P0 of the modulator, the absorption losses in the branches being neglected. Diodes PINPM1 and PINPM2 are biased by different currents. For example, the diode PINPM2 is biased by a zero current introducing theoretically a zero phase delay, and the diode PINPM1 is biased by a current Ib selected to introduce a phase delay of 180°.
FIG. 2 is a diagram illustrating the variation of the transmission rate P1/P0 of the modulator, measured at the output of the upper channel of the coupler J1, as a function of the phase difference between the waves at the inputs of the coupler J1. The transmission rate P2/P0, not shown, at the output of the lower channel, varies inversely.
The initial phase shift of 180° introduced by the diodes PINPM places the operating point of the modulator at the inflection point of a sinusoid, in a region where the linearity is best and the slope is steepest. The diode HSPM1 causes the phase to vary in a range above 180°, while the diode HSPM2 causes the phase to vary symmetrically in a range below 180°. By limiting the amplitude of these ranges, the corresponding change in the rate of transmission may be almost linear. For the transmission of digital signals, the linearity is less important, but it may be preferable that the behavior of the modulator remains balanced, which is the case under the conditions of FIG. 2.
In practice, the bias currents of the diodes PINPM are individually adjusted for obtaining the desired quiescent conditions. However, a drift of the quiescent conditions may be noticeable, particularly as a function of temperature.